1. Field of the Invention
The present invention relates to a power supply apparatus for a vehicle and particularly to a vehicle power supply apparatus equipped with a plurality of power storage devices.
2. Description of the Background Art
In recent years, vehicles equipped with a battery and a drive motor, such as an electric car and a hybrid car, are becoming a familiar sight. Equipping a vehicle with a plurality of batteries so as to achieve a longer battery-driven time period is under study.
Japanese Patent No. 3655277 discloses a power supply control system for using a high-voltage inverter-motor set with a low voltage battery module. This power supply control system for the electric traction motor includes at least one inverter for providing conditioned electric power to an electric traction motor and a plurality of power supply stages for providing DC power to the at least one inverter, each stage including a battery and a boost/buck DC-DC converter and the stages being wired in parallel. The power supply stages are controlled to maintain an output voltage to the at least one inverter.
An object of the technology disclosed in the above-described Japanese Patent No. 3655277 is to eliminate the discrepancy in state of charge (SOC) between a plurality of batteries. Although not specifically defined, use of a plurality of batteries having different input/output characteristics is not assumed, because unless the plurality of batteries have the same output characteristics, it is unlikely to make their respective states of charge uniform.
Further, in some cases, a voltage of a power storage device (including a battery and a capacitor as power storage devices) is boosted in order to allow the load to be efficiently operated, however, a voltage of a power storage device cannot be too lowered, since a boost converter has a limit of a voltage boost ratio.